In an inkjet head that jets out droplets from a nozzle and performs recording, when a nozzle column is lengthened in order to obtain a desired recording width, since obtaining jetting characteristics uniform in the width direction is very difficult, many head chips with a small width that are individually manufactured are used and arranged on a common head unit base in a zigzag pattern, thereby fabricating an inkjet head unit in which nozzles are aligned over a desired recording width as a whole (Patent Literatures 1 and 2).
Each of FIGS. 17(a) and (b) shows a conventional inkjet head unit 100 having four head chips arranged on a head unit base in a zigzag pattern so that they can form two columns. (a) is a side elevation showing a cross section of the head unit base, and (b) is a plan view showing the same from the opposite side of a nozzle face.
Head chips 200 are inserted into four attachment opening portions 301 formed in a common head unit base 300 made of a material such as SUS into a tabular shape from respective nozzle face 201 sides, and attachment flange portions 202 formed so as to protrude in a lateral direction from the respective head chips 200 abut on and are fixed to upper surface sides of side edge portions of the attachment opening portions 301, whereby the head chips 200 are arranged on the head unit base 300 in a planar manner.
Many nozzles, which correspond to, e.g., 1200 npi (nozzle per inch), are arranged on the nozzle face 201 of each head chip 200 in an array shape, and the respective head chips 200 are positioned on the head unit base 300 in such a manner that pitches of the nozzles can be an equal pitch along a Y direction in the drawing across all the head chips 200 as seen in a direction parallel to an X direction in the drawing.
It is to be noted that, in the drawing, reference numeral 203 denotes an ink manifold, and reference numeral 204 designates an external wiring member (FPC). The external wiring member 204 is connected to a surface of each attachment flange portion 202 on the opposite side of the head unit base 300.
Further, as shown in FIG. 18, each head chip 200 may be also inserted into each attachment opening portion 301 in the head unit base 300 from the ink manifold 203 side, and each attachment flange portion 202 may abut on and may be fixed to a lower surface side of the side edge portion of each attachment opening portion 301 so that each head chip 200 can be arranged on the head unit base 300 in a planar manner. In the conformation shown in FIG. 17, since each nozzle face 201 and the head unit base 300 are arranged on the same side with respect to each attachment flange portion 202 of each head chip 200, the head unit base 300 must be thinly formed in order to arrange each nozzle face 201 as close to a recording receiving surface of a recording medium (not shown) wound around and held on a drum surface 401 as possible, and there is a problem of a reduction in strength, but the nozzle face 201 of each head chip 200 and the head unit base 300 are arranged on the opposite sides so as to sandwich the attachment flange portion 202 therebetween in this conformation, and hence a thickness of the head unit base 300 can be increased and sufficient strength can be assured.
Each external wiring member 204 is placed between each attachment flange portion 202 and the head unit base 300, and the external wiring member 204 placed on the inner side is pulled out to the opposite face of the nozzle face 201 through a wiring extraction hole 302 formed in the head unit base 300.
In case of using such an inkjet head unit 100 as shown in FIG. 17 and FIG. 18 and installing it in a high-speed drum conveyer having a cylindrical rotary drum 400 to perform drawing, the inkjet head unit 100 is arranged in such a manner that its X direction in the drawing becomes parallel to a rotating direction (a circumferential direction) of the rotary drum 400. At this time, since each nozzle face 201 and the head unit base 300 have flat surfaces parallel to each other whereas the drum surface 401 of the rotary drum 400 has a curved surface, each head chip 200 has a large difference between a distance D1 from a nozzle placed on the central side of the head unit base 300 to a curved recording receiving surface (a surface) of a recording medium (not shown) wound around and held on the drum surface 401 and a distance D2 from a nozzle placed on each of both end portion sides of the head unit base 200 along the circumferential direction of the drum 400 to the curved recording receiving surface of the recording medium wound around and held on the drum surface 401 in a relationship of D1<D2, and a time required until impact of ink droplets which are to strike on the recording receiving surface of the recording medium greatly differs depending on positions of the nozzles along the rotating direction of the rotary drum 400.
Such a time lag until the impact depending on each nozzle position can be solved by finely adjusting timing for jetting out an ink from the nozzles. However, since a height position of the head unit base 300 with respect to the recording receiving surface of the recording medium on the drum surface 401 is set in such a manner that a distance between the nozzle face 201 and the recording receiving surface of the recording medium on the drum surface 401 becomes a proper distance (approximately 1 mm) which is the shortest distance D1, when the distance D2 is the longest as compared with the proper distance D1, travel of the ink droplets over this long distance D2 results in the following another problem.
FIG. 19 is a graph showing a relationship between a gap amount (a distance between the nozzle face and the recording receiving surface of the recording medium) and an impact accuracy. The measurement was carried out by keeping both the nozzle face and the recording receiving surface immovable, determining a point where a straight line vertically downwardly extended from a specific nozzle crosses the recording receiving surface as a proper impact point, and using a contactless three-dimensional measuring instrument to measure an impact position coordinate with respect to the proper impact position when a distance between the nozzle face and the recording receiving surface is increased.
As a result, it can be understood that the impact accuracy is deteriorated (an impact error increases) as the gap amount is increased. That is because, when the distance between each nozzle and the recording receiving surface increases and the traveling distance of the ink droplets gets longer, an airflow (e.g., an airflow generated by rotation of the rotary drum 400) is apt to affect, and a traveling direction is disarrayed.
Therefore, there is a problem that the ink droplets that travel the distance D2 are more affected by an airflow than the ink droplets that travel the distance D1 and an impact position deviation occurs. Such an impact position deviation caused due to an airflow cannot be adjusted by jetting timing, and it can be a cause that deteriorates image quality.
Patent Literature 3 conventionally discloses that a nozzle plate is formed into a curved surface that is bent to form part of a cylindrical shape. When the nozzle plate is curved, a distance between a surface of a cylindrical rotary drum and each nozzle can be substantially fixed, but precisely bending the nozzle plate so that it can fit to the surface of the rotary drum is very difficult, and there arises a problem of a difficulty in manufacture.
Further, a method for bending a head unit base itself in accordance with a curved shape of a rotary drum surface can be also considered, but there is also a problem that precisely bending the head unit base so as to fit to the rotary drum surface is difficult.
Furthermore, Patent Literature 4 discloses that a piezoelectric actuator is used and an inclination of a head chip with respect to a recording medium is adjusted. However, providing the piezoelectric actuator for adjustment of the inclination of each head chip leads to a problem that a configuration of an inkjet head unit is complicated, manufacture becomes difficult, and also a manufacturing cost increases.